This invention relates broadly to a method for converting solid metal oxalates, insoluble compounds having little or no industrial use, to soluble salts useful especially as catalysts or as catalyst components in processes wherein an organic feedstock, usually a hydrocarbon and most commonly a paraffin hydrocarbon, is converted to one or more oxygenated derivatives by oxidation with molecular oxygen in a liquid phase containing a homogeneous catalyst comprising a metal such as cobalt, copper, or manganese. In particular, such processes are frequently conducted employing a lower carboxylic acid, especially acetic acid, as the reaction solvent with the organic feedstock being a lower alkane of about three to five carbon atoms, although liquid phase oxidations are also sometimes employed with alkanes or cycloalkanes having up to about eight carbon atoms. Closely-related processes comprise the liquid-phase oxidation of an aldehydoalkane, such as hexanal, heptanal, or nonanal, to form the corresponding carboxylic acid.
A commonly-encountered problem in such liquid-phase oxidation systems is the progressive deactivation of the catalyst by the reaction of the metal, e.g. cobalt, with oxalic acid (formed as an undesired reaction by-product) to form the insoluble oxalate, which is catalytically inert. This oxalate tends to precipitate on the interior surfaces of processing equipment containing the liquid reaction medium and/or streams, such as distillation residues, derived from it. Alternatively, the solid oxalate may also be present as a sludge in some of these process streams.
Until recent years, cobalt in its various industrially-useful forms (e.g., its soluble salts) has been available at moderate cost, so that mixing up the catalyst lost in the manner just described has not been a serious cost item. More recently, however, there have been sharp rises in the cost of cobalt. Additionally, there is the closely-related problem of increasing uncertainty concerning the availability of this strategically-important element in the industrialized world.
The art is already acquainted with one feasible method for recovering catalytically-useful cobalt salts from solid cobalt oxalate, this method being described in U.S. Pat. No. 3,840,469 to Hobbs et al. The heart of the patented process is the digestion of solid cobalt oxalate (normally in the form of a slurry) with a solution containing calcium ions whereby, calcium oxalate being even less soluble than cobalt oxalate in, for example, acetic acid, the oxalate moiety is converted to solid calcium oxalate while the cobalt moiety is solubilized and transferred into the liquid digestion medium as a dissolved cobalt salt. This cobalt salt can then be recycled to the liquid-phase oxidation system, with or without whatever additional processing may be desired (e.g., concentration), to replenish its catalyst content. The calcium oxalate is normally discarded.
Although the process of Hobbs et al. is workable and industrially useful, it has a drawback in that the digestion step employed in it has a mass-transfer limitation in the matter of replacing cobalt ions in the solid phase with calcium ions from the liquid phase; this difficulty attends practically all such digestions as employed, for example, in many laboratory procedures and is the reason for the common practice of allowing extended periods of time at elevated temperature for such digestions to be completed. Even after extended periods of time, of course, it is also realized both in the laboratory and in industry that such digestions are often still not complete because some material remains occluded within the solid particles.
It is an object of the present invention to provide a method for recovering cobalt from a solid comprising cobalt oxalate which is workable and which does not entail the step of digesting the solid with a calcium solution requiring, as just described, the replacement of cobalt ions in a solid with calcium ions. More particularly, it is an object of this invention to provide a method for expeditiously recovering cobalt moiety from solid cobalt oxalate which has been formed in the course of the liquid phase oxidation of an organic compound, especially a hydrocarbon and more especially an alkane, in the presence of a catalyst comprising cobalt and then returning the recovered cobalt to said liquid phase oxidation as makeup catalyst. Other objects of the invention will be apparent from the following specification and claims.
Although the process of the invention is especially directed at recovering cobalt, it will also be evident to those skilled in the art that it can be applied to other metals which are employed as catalysts in liquid phase oxidation, e.g. copper and manganese, so long as those metals are capable of being dissolved by chelating agents, the process of the invention being one which employs a chelation or complexation step as will be explained. For example, U.S. Pat. No. 4,246,185, to Wood, describes separating manganese and copper catalyst moieties from alkanoic acids having six to nine carbon atoms by treatment with oxalic acid to precipitate substantially all of the copper and manganese which are present as the oxalates which can then be filtered or centrifuged out of the alkanoic acid. The present invention can be employed, if desired, to recover the copper and manganese moiety from these oxalate precipitates followed by recycle of the recovered catalyst metal moieties to an oxidation step wherein the alkanoic acids are being formed by the liquid-phase oxidation of the corresponding aldehyde precursors of the alkanoic acids.
A related use of oxalic acid precipitation to recover cupric oxalate and manganese oxalate from alkanoic acids having three to nine carbon atoms is also described in copending application Ser. No. 065,239 filed Aug. 9, 1979 by R. H. Scott et al., and the presently-described method is also applicable in recovering and recycling the copper and manganese moiety which is separated as the oxalate salts from the alkanoic acids in that process.
The primary objective of the present invention is, however, its employment in the recovering of cobalt oxalate because cobalt is of particular economic and strategic importance.